1. Field of the Invention
The present invention relates to an input/output matching circuit for a pre-amplifier of an SHF band television signal receiver.
2. Description of the Prior Art
An SHF band television signal receiver is usually constructed as shown in FIG. 1. The SHF band signal is received by an antenna 1, a signal received by the antenna 1 is supplied to a pre-amplifier 2, and an amplified SHF band signal from the pre-amplifier 2 is supplied to a mixer 4 which also receives a local oscillation signal from a local oscillator 3 and converts the SHF band signal to an intermediate UHF band signal.
An input matching circuit for the preamplifier 2 as shown in FIG. 2a or FIG. 2b, which uses a field effect transistor (FET) has been known. The circuit shown in FIG. 2a comprises a 1/4 wavelength transformer 5 constructed by a low impedance microstrip line which is connected to a gate electrode 7 of an FET 6. The circuit shown in FIG. 2b comprises an openended stub line 8 constructed by an appropriate impedance line which is connected to the gate electrode 7 of the FET 6. It utilizes a capacitive reactance of the stub line 8 and a phase rotation of a signal created between a center of the stub line 8 and the gate electrode 7. A source electrode 23 is usually grounded and a drain electrode 21 is connected to a transmission line 11 through a stub line 22.
In those prior art circuits, however, only a narrow band matching is attained because of a large input reflection coefficient. The SHF band receiver suffers from a disturbance by an image signal which is apart from the local oscillation frequency by the intermediate frequency. When an image suppression filter is connected to an input terminal of the pre-amplifier 2, the image signal can be eliminated, but since the image suppression filter further narrows the matching band, the image signal suppression is usually not effected in the pre-amplifier 2 but it is effected in the mixer 4 of the succeeding stage. In this case, if the disturbance power of the image signal is large, the pre-amplifier 2 saturates resulting in difficulty in the image signal suppression. In addition, the pre-amplifier 2 does not provide a satisfactory characteristic.